This invention generally relates to Magnetic Resonance Imaging (MRI) and, in particular, to a method and apparatus for water-fat image separation in MRI.
Fast spin echo (FSE) imaging offers fast acquisition with long repetition times (TR). FSE can be used to improve contrast of long transverse relaxation time (T2) image components with respect to short transverse T2 image components. In clinical applications, where water signals are chiefly of interest, it is desirable to attenuate or eliminate MR signals from lipids (the terms “lipid” and “fat” are used interchangeably) which tend to reduce image contrast especially in the extremities and abdominal sections of a patient. The loss of water image contrast due to lipids is also exasperated by the natural behavior of the FSE sequence to enhance the lipid signal by partial averaging the scalar J-coupling of the lipid protons. The J-coupling signal enhancement becomes progressively worse as refocusing pulses are repeatedly applied during the FSE imaging echo train. Lipid signal levels tend to be relatively intense because of partial averaging in FSE sequences especially when they have long effective echo times (TE) and short echo spacing.
Conventional approaches to attenuate lipid signals include: chemically selective radio frequency (RF) preparation, saturation, and excitation pulses, inversion recovery preparation pre-pulse such as in short tau inversion recovery (STIR) and multipoint Dixon techniques. Chemically selective RF pulses are dependent on the homogeneity of the main magnetic field. For successful application of chemically selective RF pulses, magnetic field homogeneity of one (1) part per million [ppm] or better is typically needed to effectively attenuate lipids during image acquisition. Such high levels of main field homogeneity are not always achievable over the entire field of view (FOV) being imaged. Even where shimming of the main magnetic field provides field homogeneity under 1 ppm, small field inhomogeneities can occur in certain anatomical areas being imaged. For example, a discontinuity of magnetic susceptibility of several ppm can occur at tissue-to-tissue and tissue-to-air interfaces. Furthermore, at mid and low magnetic field strengths, the actual frequency difference between water and lipid signals is so small that impractically long RF pulse lengths are needed to discriminate between the water and lipid signals.
An inversion recovery preparation pre-pulse, e.g., STIR, to prepare the subject region for acquisition is simple and easy to implement. A difficulty with STIR is the loss of signals from tissues that have longitudinal relaxation times (T1) which are similar to the T1 of the attenuated lipids. At mid-field strengths, typical values of T1 for muscle, brain, cerebrospinal fluid (CSF) and lipids are: 450, 600, 3500 and 220 milliseconds (ms) respectively. Although lipids have the fastest relaxations, the recovery curve after an inversion pre-pulse at an evolution time of about 100 to 150 ms removes significant amounts of signal from the water components which reduces the overall conspicuity of many features of clinical importance.
Phase sensitive methods have been applied to distinguish water and lipid signals. These methods rely on the phase increment of the water signals relative to lipid signals in a homogeneous field. Within the small volume of a single image volume element (voxel) the main field can be treated as uniform even though it is not so over the entire image field of view. The phase increment α is given in radians by the following relationship (1):α=2π*ΔF0*Δt   (1)where ΔF0 is the water-lipid chemical shift in hertz (Hz) and Δt is the evolution time in seconds (sec). The evolution time is measured from the echo formation to a point where the first order interactions are canceled out, such as at the nominal time TE or at a multiple of TE for each echo in the sampling window. In a typical fast spin echo sequence, TE is also the time distance between refocusing pulses and TE/2 is the time between the 90 degree excitation and the first refocusing pulse.
When echo formation occurs at a time different from the mid-point between the refocusing pulses, a water-lipid phase difference is generated for every voxel in the imaging volume. The water-lipid phase difference may be used to distinguish the water and lipid signals, and thereby facilitate attenuation of the lipid signals. The water-lipid phase difference may be relatively small or be obscured by inhomogeneties in the static magnetic field. In the past, identifying and using the water-lipid phase difference to attenuate lipid signals has been problematic. There remains a long felt need for improved phase sensitive methods to suppress lipid signals and thereby improve MR images.